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PreAmp output transformers

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regal

Member
2004-01-04 8:41 pm
MD
What are the drawbacks of preamp output transformers? Say a typical 5k:600 OPT feeding an amp with a 10k impedance. Now the reflected impedance on the preamp tube is 80k. How does this work out? would seem such a shallow loadline as to be no power left? With the 5842 spud amp I have with the 10k input impedance an OTL analog stage of the DAC I'm building is a bit outof the question, so I am looking for opinions on the use of line level output transformers.
 
This may be a different way of looking at it, but I've been getting really good results with the Hammond 126C which is cheap enough at thirty something dollars (£26 in the UK). This is 10k:10K, 1:1. I'm guessing that you could increase the input impedence of your amp, since I'm guessing it has a pretty typical tube first stage. That would leave you free to use a preamp tube of up to 5K plate impedence. The OPT is 1:1 so you don't need much gain. I'm using a 10Y into the 126C, which is pretty much the king of preamp tubes along with the 26 which won't suit the 126C (impedence too high). Other possible choices in directly heated triodes are 31, 46 and 71A. Plenty of indirectly heated tubes though they won't sound so good.

That's my take - an "alternate take" as they say in the recoding industry.

andy
 

45

Member
2008-12-18 2:29 am
UK
What are the drawbacks of preamp output transformers? Say a typical 5k:600 OPT feeding an amp with a 10k impedance. Now the reflected impedance on the preamp tube is 80k. How does this work out? would seem such a shallow loadline as to be no power left? With the 5842 spud amp I have with the 10k input impedance an OTL analog stage of the DAC I'm building is a bit outof the question, so I am looking for opinions on the use of line level output transformers.

The reflected impedance will be dominated by the transformer reactance at low frequency and losses at high frequency. More likely the reflected impedance as a function of frequency will look like a "mountain" profile: a (more or less) broad peak around 1K. The frequency response at low signal level can still look reasonably flat but the distortion can change quite a bit (as a function of frequency). So it also depends on the output level you are looking for.
If a transformer is specified for 5K primary impedance that's the order of magnitude one should expect at low frequency. In general it depends on the primary inductance. There are cases where the OPT has generous primary inductance and if you load it to get 10K it will work very well or cases where the 20Hz reflected impedance is lower than the nominal value even using the nominal load.

45
 
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